1. Field of the Invention
The present invention relates to a method of producing a resin-sealed electronic device.
2. Related Arts
In recent years, as shown in FIGS. 15 and 16, an electronic component 21 such as a semiconductor device or the like is directly mounted on a circuit board 22 to comply with miniaturization and densification requirements for an electronic device. In more detail, as shown in FIG. 15, is a case where the electronic component 21 is mechanically bonded to the circuit board 22 by adhesives 23, and thereafter, it is electrically connected to electrodes formed on the circuit board 22 by bonding wires 24. Further, as shown in FIG. 16, the electronic component 21 is bonded to the circuit board 22 by utilizing solder (conductive adhesives, or a conductive adhesive film) 25 so that mechanical fixation and electrical connection between the electronic component 21 and the circuit board 22 are attained at the same time. In either case, the electronic component 21 is sealed by sealing resin 26. The objects of the sealing are to improve junction reliability between the electronic component 21 and the circuit board 22, thermal stress-resistant reliability and moisture-resistant reliability of the electronic component 21. Further, the other object of the sealing is to mechanically protect the electronic component 21. Here, to attain the above-mentioned objects, it is necessary that the sealing resin 26 reliably adheres to the bonding wires 24, to the bonding member such as the solder 25, as well as to the surface of the circuit board 22.
In general, contaminants and the like are less likely to attach to the surfaces of the bonding wires, the solder, and the electronic component. Therefore, adhering reliability of the sealing resin relative to the bonding wires, the solder, and the electronic component is typically not problematic. As opposed to this, in many cases, adhering reliability of the sealing resin relative to the circuit board is not sufficient. In particular, in the case where the circuit board is made of resin, some components included in the circuit board seep out on the surface of the circuit board, thereby deteriorating the adhering reliability of the sealing resin to the circuit board.
In addition, a printed circuit board including paper-phenol base material or glass-epoxy base material is often employed as the circuit board because of its characteristics and cost. On the printed circuit board, as shown in FIG. 17, an electronic component is mounted through a patterned copper layer 30 and a solder resist layer 27. The solder resist layer 27 is coated on the copper layer 30 in order to prevent the oxidation of the copper layer 30 and to prevent the adhesion of solder, which is used to mount the electronic component, to unnecessary portions. In this structure, an adhering area of the sealing resin 29 relative to the solder resist layer 27 is large, and the adhering strength of the sealing resin 29 relative to the solder resist layer 27 is greatly affected by the material of the solder resist layer 27.
To form the solder resist layer 27, liquid resist material is coated on the circuit board 22 to cover the copper layer 30, and then, is hardened by heating or ultraviolet irradiation. When coating the liquid resist material, however, foam is liable to occur so that pin-holes are caused in the solder resist layer 27. To prevent this problem, it is known that a silicone family surfactant 28 (indicated by white triangles in FIG. 17) such as polydimethylsiloxane or the like is added to the solder resist layer 27 as an antifoaming agent. However, this silicone family surfactant 28 deteriorates the adhering strength with respect to the sealing resin.
JP-A-58-15261 and JP-A-60-1220 propose methods for improving characteristics of the sealing resin. However, the adhesive properties of the sealing resin relative to the solder resist is not sufficient. For example, when performing a thermal shock cycle test or a high-temperature and high-humidity storage test, in many cases, separation of the solder resist from the sealing resin occurs. The separation of the sealing resin is accompanied by breakdowns of the electronic component, breakdowns of the connecting portions between the electronic component and the circuit board, and invasion of water into the separated portions which causes corrosion. As a result, reliability of the electronic component is largely deteriorated.
As opposed to the above mentioned problems, JP-A-6-232457 proposes a method that an electronic component is covered by coating resin having good wettability in advance and sealing resin covers the coating resin. However, this method needs processes of coating the coating resin and drying it, thereby resulting in increases of the processing cost and material cost.